A large amount of optical information is transmitted in a multi-level phase shift keying modulation format. An optical receiver including a 90-degree hybrid may be used to demodulate multi-level phase shift keying signals. The 90-degree hybrid exhibits output forms having different branching ratios based on the phase modulation state of signals. The monolithic integration of the 90-degree hybrid on a semiconductor substrate reduces the size of the 90-degree hybrid and allows the 90-degree hybrid to exhibit broad wavelength bandwidth characteristics.
For example, a 90-degree hybrid having a monolithically integratable optical waveguide structure includes a 4:4 multimode interference (MMI) coupler disclosed in E. Pennings, et. al., “Ultracompact, all-passive optical 90°-hybrid on InP using self-imaging”, IEEE Photonics Technology Letters, Vol. 5, No. 6, pp. 701-703, June 1993. In the 4:4 MMI coupler, two channels having central axial asymmetry out of four input channels on the input side are appropriately selected to obtain a 90-degree out-of-phase relationship due to the mode interference effect in an MMI region.
Related art is disclosed in, for example, Japanese Laid-open Patent Publication No. 2010-171922; E. Pennings, et. al., “Ultracompact, all-passive optical 90°-hybrid on InP using self-imaging”, IEEE Photonics Technology Letters, Vol. 5, No. 6, pp. 701-703, June 1993; and D. Hoffmann, et al., “Integrated Optics Eight-Port 90° Hybrid on LiNbO3”, Journal of Lightwave Technology, Vol. 7, NO. 5, pp. 794-798, May 1989.